The present invention relates to a novel adaptive filter and to operation of the same.
In a typical telephone connection, at each end of the network a four wire (4W) segment is joined with a two wire (2W) segment by a hybrid circuit. Impedance mismatch in a hybrid circuit may cause the 4W receive path signal, e.g., a far end speaker's voice signal, to be reflected onto the 4W transmit path as an echo signal. The hybrid circuits and two pairs of wires in the 4W segment form a loop around which echo signals propagate. Normally, at the ends of the network the 4W receive path signal is at a higher level than its echo on the transmit path, since there is loss across the hybrid circuit. This loss, which varies from one hybrid circuit to another, is a function of frequency and called transhybrid loss (THL).
It often is desirable to insert gain in the 4W paths to compensate for losses in the 2W segments, in which case the hybrid circuits provide loss and the gain stages provide gain. If, at any frequency, the inserted gain is greater than the sum of the losses in the two hybrid circuits, an unstable condition will exist. Once such a passband frequency is excited by a signal in either 2W path, it will continue to grow as it propagates around the loop, until it saturates at some level. This unstable condition is known as singing, and is an undesired self-sustaining oscillation at a frequency in the passband of the system. To avoid singing, either the gain must be decreased or the hybrid circut losses increased. Decreasing the gain may result in an unacceptable quality of service, so increasing the THL is the more desirable alternative of the two.
A common technique for increasing the THL of a hybrid circuit is to use an adaptive transversal filter with the circuit, resulting in a so-called adaptive hybrid. The adaptive filter operates much like an echo canceller, in that it generates an estimate of an impulse response of the end path coupling the 4W receive and transmit segments, and through a process including convolution of the estimated impulse response and receive path signal, develops an estimate of the echo on the transmit path. The echo estimate is subtracted from the transmit path signal, resulting in a residual signal that is used to update the estimate of the impulse response in a manner to reduce the power or absolute value of the residual signal. However, because of noise on the end path, which includes any near end speech signals, it often happens that the adaptive filter does not converge sufficiently to prevent singing on the network at desired gain levels.